Advances in semiconductor device performance are largely attributable to reductions in device dimensions. Initially, the photolithographic and other micro-electronic fabrication techniques enabling such device scaling were not amenable to inductors, capacitors and other passive components often employed in semiconductor devices. Nevertheless, inductor design eventually evolved to take similar advantage of the advances in micro-electronic fabrication processes and experience scaling to a degree similar to that of active devices.
By way of example, inductors are now typically created on the surface of a semiconductor device substrate in a spiral shape created in a plane that is parallel to the substrate surface. The inductor may have several of these spiral-shaped coils as necessary to attain adequate inductance values. Unfortunately, conventional methods used to create the spiral-shaped inductors may produce inductor devices having limited performance.
The performance parameter of an inductor is typically indicated by its quality factor, Q. The quality factor is defined as the ratio between the energy stored in the reactive portion of the inductor and the energy that is lost in the reactive portion. In general, the higher the quality factor the better the performance of the inductor.
In some circumstances, an ultra-thick metal (UTM) process is utilized to fabricate inductors. However, the UTM process is rather complex and requires a large chip area. In addition, the UTM process may limit or constrain the pursuit of a high quality factor.